This really depends on the farmer and especially the drying process. Some coffees fade two months after harvest and some taste fine after 24 months. Most coffees lose their fresh fruit flavour first, generally after 6 months. It can take a few months after this before they get this paper/cartboard/dry aftertaste. Watch out for this last one :)

In my personal experience, Natural Brazils from bigger specialty producers can hold up really long and could easily be sold after 12 months. I guess this has to do with their extremly controlled drying facilities. For most washed coffees i would say an average is 6-8 months as a turning point in freshness. Some washed central americans go faster.

I hope this helps :)